This invention relates to reducing the flow assisted corrosion of carbon steel components exposed to flowing water having a low oxygen content of about 150 parts per billion or less, herein referred to as low-oxygen water. As used herein the terms "parts per billion, " or "ppb" is of mass. As used herein the term "flow assisted corrosion" means the effect of fluid flow that accelerates general corrosion by increasing the rate of mass transport of reactive species to and from the metal surface, and acceleration or increase in the rate of corrosion caused by the relative movement between a corrosive fluid and the metal surface.
Low-oxygen water can be found in a variety of known apparatus, such as water deaerators, nuclear reactors, and in steam driven central station power generation systems. Nuclear reactors are used in central-station electric power generation, research, propulsion, and for dual purposes thereof. A reactor pressure vessel contains the reactor coolant, i.e. water, which removes heat from the nuclear reactor core. Piping circuits carry the heated water or steam to the steam generators or turbines, and circulated water or feedwater back to the vessel In nuclear reactors, the low-oxygen water is typically found in the feedwater, and the feedwater piping circuits are most subject to the flow assisted corrosion Corrosion products in reactors cause problems relating to radiation level, radioactive waste, and heat transfer, so control of corrosion products is very important.
Carbon steels are extensively used in low pressure and high pressure turbine sections, and feedwater heaters in nuclear power plants including many ancillary components of the steam-water circuit, such as moisture separators, and reheaters. In these components high flow velocities under single-phase, water, or two-phase, wet steam, conditions prevail.
Damage associated with flow assisted corrosion of such power plant components generally occurs at locations where there is severe fluid turbulence adjacent to the metal surface, either from high fluid velocities, e.g., greater than 2 meters per second, or due to the presence of features such as bends or orifices that generate high local turbulence levels. A thin layer of oxide; one micron or less, is normally present on the corroding surface, but the rate of penetration in these localized areas deprived of a characteristic double layer oxide film can reach values as high as 0.1 to 10 millimeters per year. Such rates of metal removal are unacceptable in power plants which have a design lifetime of 30 to 40 years, but even significantly lower rates of attack may generate undesirable high concentrations of corrosion products in the water circuits.
Iron release rates from carbon steel decrease by up to two orders of magnitude over the temperature range of 38.degree. to 204.degree. C. with increasing oxygen concentration from 1 to 200 parts per billion. It is expected that flow assisted corrosion rates follow this type of behavior. For example, it has been shown that a low alloy steel containing 0.5 percent molybdenum exposed to water at 120.degree. C., exhibits a decrease of more than four orders of magnitude in the flow assisted corrosion rate at oxygen concentrations above 150 parts per billion up to 500 parts per billion. In agreement with this, many reported cases of flow assisted corrosion damage under single-phase conditions occur within the temperature range of about 50.degree. to 230.degree. C., whereas under two-phase flow the range is at higher temperature of about 140.degree. to 260.degree. C.
Flow assisted corrosion of carbon steel in neutral low-oxygen water was investigated by M.Izumiya et al., "Corrosion and/or Erosion in BWR Plants and their Countermeasures," Water Chemistry and Corrosion Problems in Nuclear Power Plants, International Atomic Energy Agency, SM-264/4, 1983. Referring to FIG. 1, reproduced from the above referenced disclosure, the effect of dissolved oxygen on the flow assisted corrosion rates of a carbon steel comprised of about 0.15 weight percent carbon, 0.21 percent silicon, 0.69 percent manganese, and 0.013 percent phosphorous, in neutral water at 100.degree. C. is shown. The carbon steel corrosion rates are significant when the water contains less than 20 parts per billion oxygen, but addition of more than 40 parts per billion oxygen reduces corrosion substantially. Oxygen gas addition to the feedwater of boiling water reactor plants has been practiced to reduce the iron released to reactors and the radiation levels on recirculation piping.
Although no comprehensive model of flow assisted corrosion which can fully describe the effect of the variables mentioned above has yet been developed, it is believed that the corrosion rates observed at high flow velocities are due to enhanced dissolution of magnetite leading to accelerated metal loss as iron oxidizes to replace the dissolved film. Additional information about flow assisted corrosion can be found, for example, in "Erosion-Corrosion in Nuclear Power Systems-An Overview," G.Granolino, Corrosion 87, San Francisco California, paper no. 86.
It is an object of this invention to provide a method for reducing the flow assisted corrosion of carbon steel components exposed to flowing low-oxygen water.